The things is that they do have the right to do so. If they were to simply, unilaterally declare which candidates they're going to put forward that too would be perfectly legal. There is no law mandating primaries. The parties are Independent organisations and how they pick their candidates is entirely up to them. If you were to create a brand new party and pick which candidates you're putting forward by drawing straws that would be perfectly legal too. Both parties just choose to hold primaries because for one thing it picks a candidate more likely to win and secondly it obscures how much democracy has been undermined in the US. The systems have been rotting well before Trump.

If that were true Analilia Mejia wouldn't have beaten Tom Malinowski. She did.

If that were true, Cuomo would've beaten Mamdani. He didn't.

If that were true Janet Mills would be winning her primary. She's not.